This invention relates to the production of polyvinyl butyral sheet and particularly to the formation of rolls of the sheet material for storage and transportation.
Polyvinyl butyral (or PVB) is a very important tough plastic that finds its major utility in the production of interlayers for laminated safety glass. When a sheet is laminated between two plies of glass, the resulting laminate is extremely resistant to penetration by an impacting object. This has led to very wide use of the plastic in sheet form for the production of car windshields, security glazing and architectural laminates.
Most frequently the manufacturer of the PVB sheet does not make the laminates himself so that the sheet has to be formed into rolls, stored and transported before it is used.
It is found however that when rolls of PVB sheet are unrolled by the laminator they encounter two serious problems. The first is that towards the center of the roll, for approximately the last ten percent of the sheet on the roll in fact, a serious "blocking" problem is encountered. "Blocking" is a phenomenon displayed by a number of polar thermoplastic sheet materials that causes sheets placed in face-to-face contact under pressure to tend to stick together. A roll of conventional, untreated, thermoplastic sheet is said to be blocked when it is no longer possible to unroll it manually by pulling on the free end without distorting or wrinkling it. Generally manual pull-off forces, as registered by a strain gauge, can be up to about 13.6 kg. Blocking is a particular problem with PVB sheets and is very severe in conventionally wound rolls unless the sheet is dusted with a powder to inhibit adhesion of adjacent plies.
The second problem referred to above is that of retarded strain recovery. The PVB is commonly produced and wound onto rolls in an on-line fashion with the result that the sheet bears within it strain imparted to it during the production process. Further strain and differences in strain are imparted as the successive layers are wound on the relatively non-compressible core. As a result of the accumulated hoop tension effects of outer layers and the resistance to compression of the core, differences in retarded strain recovery can be objectionably high. When the PVB sheet is placed between hot glass laminae in the formation of a laminate the sheet relaxes and shrinks often by as much as fifteen percent or more. Since the amount of shrink varies it is difficult to compensate for this effect accurately.
These and other problems have been minimized by the present invention which provides a means of winding a PVB sheet to produce a roll with essentially no blocking problems and with minimal variation in shrink tendency.
The invention, in a further aspect, provides a means of winding a PVB sheet printed with a color gradient so as to eliminate strike off of dye used to print the gradient at a line corresponding to the leading edge of the initial lap of sheet on a core roll.